There is substantial evidence that some hormone-dependent human breast cancers can synthesize androgens and/or estrogens from extracellular precursors. However, investigations into this and other potentially important aspects of sex steroid regulation have been hampered by difficulties establishing such tumors in long term tissue culture. We have observed that MCF-7, an established line of hormone-responsive human breast cancer cells, also has sex steroid biosynthetic capability. Furthermore, our studies suggest that both the formation and action of estrogens in this cell line may be subject to androgenic regulation. The objectives of this proposal are to utilize MCF-7 cells to 1) further characterize the androgenic regulation of estrogen biosynthesis and action 2) determine to what extent androgen formation and action may be modified by estrogens, and 3) to test the hypothesis that the hormonal regulation of human breast cancer cells can be mediated by sex steroid derived from intracellular biosynthesis. These studies will involve the measurement of enzyme activities and steroid hormone receptors in cells incubated with androgens, estrogens, or their precursor steroids with and without various enzyme inhibitors, antiestrogens, and antiandrogens. These studies are likely to provide a series of new insights into the hormonal regulation of human breast cancer cell growth which should eventually lead to the development of improved breast cancer therapies. Since it is likely that there are fundamental similarities between the hormonal regulation of malignant and healthy tissues, our data may also provide a better understanding of the basic mechanisms of steroid hormone action.